Cellular responses to ionizing radiation (IR)-induced DNA double strand breaks (DSBs) involve activation, via the phosphatidylinositol 3-kinase-like kinase (PIKK) ATM, of cell cycle checkpoint proteins that delay cell cycle progression to allow DNA repair and to preserve chromosomal integrity. Responses to DNA DSBs induced by the radiomimetic enediyne C-1027 deviate from this model. For example, IR induces ATM-dependent, and C-1027, both ATM- dependent and independent, responses. Furthermore, treatment with equi-cytotoxic levels of IR or C-1027 results in either a very low (2.9%), or an extraordinarily high (92%), percentage, respectively of mitotic cells showing aberrant chromosomal recombination. FISH analysis revealed that C-1027-induced chromosomal aberrations are frequently associated with disruption of telomeres. The mechanism(s) behind these unique responses to the antitumor drug, C-1027 should help extend our understanding of DNA damage responses to DSBs. First, pivotal checkpoint pathway proteins involved in the response to C-1027 will be identified, with emphasis on both ATM-dependent and independent responses. Next the conditions under which C-1027 induces extensive chromosomal fusions, characterized by aberrant end-joining and fragmentation, will be identified. The NHEJ DNA binding protein Ku, which not only is involved in repair responses but also plays a role in regulating chromosomal fusions, will also be examined for its role in C-1027 induced aberrant end-joining. Finally, we will test whether C-1027, which induces DSBs preferentially within a GTTA motif, targets the telomere tandem repeat sequence (GGGTTA) and contributes to aberrant end-joining by inducing telomere dysfunction. Other enediynes with cleavage characteristics different from C-1027, will be tested selectively to provide additional mechanistic insights into which properties of C-1027-induced damage are associated with the DNA damage responses. The specific aims will test the following hypotheses: 1. C-1027 is unique compared to other enediynes and to IR in that it induces both ATM-dependent and independent DNA damage responses. 2. C-1027 is unique compared to other enediynes and to IR in that it causes extraordinarily high levels of rearranged chromosomes. 3. C-1027 is unique compared to other enediynes and IR in that it targets telomeres.